Originally the public switched telephone network (PSTN) was a network of fixed-line analog telephones, called a plain old telephone system (POTS), which used switchboard operators to connect a calling telephone to a called telephone. As the network grew, central offices emerged that employed mechanical switching arrangements that would automatically connect the calling telephone to the called telephone without human intervention. These circuit-switched interconnections were made through cabling that used a collection of dedicated twisted pair copper wires.
Today, the PSTN is almost entirely digital in structure and includes mobile as well as stationary user stations. The structure of the PSTN is perhaps best viewed in terms of its customer access networks (CANs), which connect customers to local exchanges, and interexchange networks (IENs) that link these local exchanges in major cities to other cities by trunk links. Analog signals from calling units associated with the CAN are digitized at the sending exchange and time division multiplexed to allow multiple calls to use a single trunk link simultaneously. At the receiving exchange, these digitized signals are demultiplexed and converted to analog form again for transmission through the CAN to the called unit.
Typically, CANs connect customers to local exchanges using copper wire. However, trunk links and international links for IENs often use digital transmission over optical fiber due to the high aggregate data rates required (up to 2.4 gigabits per second on main trunk routes). The synchronous digital hierarchy (SDH) is a standard architecture, having a primary bit rate of 155 megabits per second, being used to accommodate the higher data rate requirements associated with IENs. However, the enhanced data rate capability of SDH equipment is achieved at a considerable cost of installation and maintenance and the physical inconvenience of laying additional cabling.
Currently, circuit-switched networks use various SDH technologies such as T1/E1/J1 Line Interface Units (LIUs) to provide internal interconnections and to connect to other routers such as media gateway routers. SDH standards dictate that these T1/E1/J1 LIUs contain packet overhead frame structures for identification purposes. Additionally, gateway routers then use additional IP/UDP or other required packet headers with these frames thereby adding significantly to overhead on the transported data. These applications required heavy frame structure, byte-alignment, and synchronous signaling, which are non-transparent header additions that reduce data transfer capabilities. Additionally, they also require the use of wireline infrastructure such as SDH approved cables and equipment thereby making applications problematical for many geographical terrains.
Accordingly, what is needed in the art is an enhanced way to provide trunk links that overcome these limitations.